1. Field of Invention
The present invention relates to composite photography, and more particularly to a matte process employing backing screens having improved chromatic actinic stimulus for color difference composite photography, cinematography, videography and solid state digital imaging.
2. Art Background
In motion picture production, it is often impractical, impossible or simply uneconomical to place actors in the specific environments to be depicted. To resolve this problem, various techniques have evolved over the years to composite such scenes from separately filmed "elements." The patent literature contains a great deal of teaching in this field. A comprehensive discussion is to be found in my prior patents. See U.S. Pat. Nos. 4,417,791, 4,548,470 and especially 4,629,298. Reference is also made to the American Cinematographer Manual, Seventh Edition (hereinafter "the ASC Manual"), pp. 430-466, with particular emphasis on the section titled "Screen Types and Lighting" pp. 434-437. With these references in mind, the present discussion will be confined to a summary of the evolution of traveling matte technique.
The earliest efforts at composite photography generally resorted to animation, as in Georges Melies' "Trip to the Moon" (1902). Thereafter, techniques such as the "held/take" process were utilized, in which a scene was shot with predetermined areas of the successive frames blocked off with an opaque "matte" in order to preclude exposure thereof. The unexposed portions of the successive frames were thereafter exposed to the desired foreground subjects, with the background areas "matted" to protect the previously recorded latent images. Essentially the same process is used to incorporate a painting which depicts a distant, dangerous, or totally alien scene against which the actors are to appear; this is known as matte painting. In order to depict actors or other foreground subjects moving in front of the desired background scenes, it became necessary to produce "mattes" that would change from frame to frame, or "travel." Various techniques were developed over the years to accomplish this.
Early processes relied upon contrast alone, the foreground action being filmed against a jet black backing and the resulting image being printed through several generations of high contrast film stock or alternatively, having the image chemically "intensified" until a matte was produced. One example of this technique is described in U.S. Pat. No. 1,273,435 to Frank Williams in 1918.
The results obtained by this technique were generally quite poor, due to the inevitable distortion produced by the multiple reversals or the intensification which result in "haloes" or "fringes" occurring between the scene elements. Efforts to address these problems led to the exploitation of the chromatic response of black and white photographic film and resulted in the Dunning-Pomeroy process (U.S. Pat. No. 1,613,163 to Carrol D. Dunning, 1927) and another Williams process (U.S. Pat. No. 2,024,081, Dec. 10, 1935). With the advent of color film recording, notably the Technicolor process, the chromatic based systems began to proliferate. (See U.S. Pat. Nos. 2,693,126, and 2,740,712 to W. E. Pohl.).
The fundamental concept that makes it possible to derive a matte from a polychromatic photographic image is based on the fact that the superimposition of positive and negative images will cancel each other out and yield an opaque image. Thus it follows that if a given portion of the image is comprised of a pure monochromatic object, e.g., blue, this portion will appear as light in a print of the film record that is sensitive to blue and dark in prints of the film records that are not sensitive to blue, i.e. the red and green records. Therefore, if the red negative record, in which the "blue" object appears light, is superimposed with the blue positive record, in which the blue object also appears light, the blue object will remain the only significant "light" object in the scene, all polychromatic portions of the scene having canceled each other out to yield an opaque image. It is then straightforward to produce a set of positive and negative high contrast "mattes" and employ these to print, in succession, the foreground and background elements of a composite scene.
With the advent of monopak color photographic film it became possible to devise the ever more sophisticated color difference traveling matte techniques exemplified by Petro Vlahos' U.S. Pat. No. 3,158,477. As the compositing technology evolved to produce more convincing results, the requirements for the original photography of the "bluescreen" element became increasingly severe. Much ingenious attention was focussed on this area, and some of the results achieved have been recognized with patents and Academy of Motion Picture Arts and Sciences Scientific and Engineering Awards. Among these are: Eastman Kodak for color negative EC 5295, a film designed expressly for Bluescreen traveling matte photography (1987), the Stewart Traveling Matte Transmission Bluescreen backing (1964), the Blue-Max Blue Flux Front Projector (1984) (U.S. Pat. No. 4,629,298) and the Reverse Bluescreen Process (1983) (U.S. Pat. No. 4,417,791). The ultimate sophistication in traveling matte image acquisition is achieved with the Reverse Front Projection process described in the American Cinematographer Manual, p. 457, which solves a host of problems. This technique provides great control over chrominance and luminance and essentially cancels any prospect of "spill" and unwanted reflections.
The latest advances in compositing technology exploit the capacity of computer image manipulation processes and digital film scanning and printing techniques, and have vastly expanded the application and efficacy of composite photography. The catalogue of Petro Vlahos' patents in the field traces the development and increasing sophistication of electronic compositing. While the below listed patents describe the electronic hardware embodiments of the process, these have all now been implemented in computer software for digital electronic composites:
U.S. Pat. No. 3,595,987 PA1 U.S. Pat. No. 4,007,487 PA1 U.S. Pat. No. 4,100,569 PA1 U.S. Pat. No. 4,344,085 PA1 U.S. Pat. No. 4,409,611 PA1 U.S. Pat. No. 4,589,013 PA1 U.S. Pat. No. 4,625,231
In Ultimatte (Vlahos) matte extraction logic, as applied to digital film composites today, the process (while still quite similar), is freed from confinement to the Blue record and readily incorporates garbage and window mattes without any compromise of the finely detailed continuous tone feature.
The starting point for a digital blue or green screen color difference composite is a matte generated by subtracting the value of one color from the value of another for each pixel in the image. (Whether this is accomplished through software or through analog video circuitry, the net effect is the same.)
With Blue logic, the raw matte is a greyscale image whose value at each point is simply the amount by which Blue exceeds the higher of the other two colors. The result is a matte which is dead black anywhere Blue is less than Red or Green and some shade of grey wherever Blue is predominant primary color.
This matte is subjected to a variety of adjustments before it is used to process the foreground and background images, but the crucial point is that the matte is generated from the absolute levels of the color components for each pixel. A pixel having values of 200 Blue, 100 Green and 100 Red will yield a pixel with a value of 100 in the matte while a darker pixel of the same hue with values of 100 Blue, 50 Green and 50 Red will yield a matte value of 50.
In other words, the Ultimatte electronic or digital color difference matting process is a function of the luminance or brightness of the backing as well as the chrominance (hue) or purity of its color and the uniformity or consistency of the matte field.
What emerges quite clearly from this description of how the Ultimatte (and other comparable matte extraction programs) work is that chrominance (the purity of the backing color), luminance (the brightness of the backing color) and uniformity (the lowest possible variations in chroma and luminance) are all crucial to the process of creating a matte and to the subsequent composite image.
In 1992, Eastman Kodak Company developed an effective film digitizing scanner and a complementary film printing laser. These systems and others produced by different manufacturers provide extensive software programs covering every facet of compositing and image manipulation technique. It is now possible to create composites containing an infinite number of elements without any degradation of image quality from the original digital scan through to the laser film output. The most subtle image attributes can be retained, including extremely fine detail such as strands of hair, as well as the all important motion blurred edges of moving objects. Translucent objects such as glass, water and smoke may now be routinely rendered in totally convincing "seamless" composites.
The extremely high demands such sophisticated computer compositing programs make on original traveling matte photography can demonstrably be met by the previously described technology such as Blue-Max (R) front projection, Reverse Front Projection and the like. However, these techniques, as sophisticated in their way as the computer programs, are technologically complex and time consuming to employ. The immense proliferation of composite photography occasioned by the facility and efficacy of digital composite technology require the development of simple, effective and economical techniques for achieving the original image or "bluescreen element."
Throughout this discussion, the process has been described by the term "Bluescreen." This is explained by the fact that for most of the history of the process, the backing color of choice, and frequently of necessity, has been blue. While it is possible to perform photochemical optical traveling matte composites using any primary color backing, there has been a persuasive technological rationale for confining the process to the blue version. With the advent of the digital electronic processes described above, however, the range of backing colors is expanded to include all the primaries and indeed, their complements. Further, freed from the constraints entailed in the photochemical process, the advantages to be found in matting on the green record can now be readily accessed. A full discussion of the relative merits of blue versus green is not warranted here beyond the mention of some of the more obvious attributes involved.
In monopak color film, particularly that balanced for Tungsten light, the Blue sensitive record is, of necessity, comprised of a fast, and hence, grainier record than either Red or Green. This is due to the relative paucity of blue light available in the Tungsten spectrum. In fact, the film emulsion designers make a major effort to provide the green record with the highest possible image attributes. Thus many aspects of perceived image quality such as resolution, tone scale, acutance, and so on are delivered to the viewer via the green record of the monopak color film. (A similar situation also prevails in video imaging devices, where the bandwidth assigned to the respective color channels was derived from the visual response of the human eye; thus the Green channel is some 59%, versus approximately 30% for the Red and only 11% for the Blue.) In most photochemical compositing techniques, this attribute of the green record was superfluous, as the "matte" record, usually blue, was reduced to a high contrast black and white matte. By contrast, in a sophisticated digital electronic computer compositing system, the matte record is rendered as a continuous tone black and white image. Actually, the matte should no longer be thought of as an "image," but rather as a signals matrix containing the instructions for combining the relative proportions of both foreground and background picture elements (or pixels) which will comprise the eventual composite image. This is now known as the Alpha channel. For a comprehensive discussion of the Alpha channel, see "Compositing Digital Images," Thomas Porter and Tom Duff, in Computer Graphics, Vol. 18, No. 3, p. 254, Jul. 1984, in which the concept was introduced. Mattes produced using this technology are capable of readily reproducing the most subtle image attributes including translucent objects such as smoke and water, filmy fabrics, and, importantly, the edge blur of rapidly moving objects in the scene, as well as shadows. Such attributes were relatively much harder to render in photochemical optical composites, though by no means impossible, when a highly skilled practitioner of the art was involved.
The most significant issues noted above are those of "motion blur" and "shadows." In these situations the compositing system will be combining proportions of both foreground and background portions of the scene together. It is desirable that a shadow cast by the foreground scene onto the background matte field will retain enough image density to record in the Alpha channel, or matte, as a smooth quiet signal. The same is true for the reduced background signal occurring in the area of "motion blur" when a rapidly moving portion of the foreground subject is partially, though not completely, obscuring the background matte field. A great deal of filmed traveling matte footage is transferred to video via a telecine device, the leading such device in the industry being the Rank Telecine. This is essentially a flying spot scanner device employing a CRT source together with optics, such that a film image frame is scanned by the CRT "spot" whereby each pixel is coded into its component parts and stored as data. The device is handicapped by the fact that the CRT phosphors employed are essentially green in color, requiring excessive amplification of the relatively weak signal derived from the blue record of film. Thus the relative grainy record of Tungsten balanced negative film is exacerbated by the excessive electronic amplification resulting in what is termed "noisy" mattes. Quite obviously, deriving a matte signal from the fine grain green record of the same film illuminated by an essentially green phosphor CRT tube will produce an electronically very "quiet" matte.
Another, small advantage of matting on the green record is derived from the fact that the optics of the camera are designed mainly around the green portion of the spectrum and, assuming the camera has been properly focussed, the very best focus will occur for the green record, with very deep red objects suffering slightly by comparison.
Further discussion on the relative merits of Blue versus Green may be found in Ultimatte Technical Bulletin No. 2, "Green or Blue--Selecting a Backing Color for an Ultimatte Composite." (Published by the Ultimatte Corporation, manufacturer of Petro Vlahos' inventions previously referred to.) After a discussion of the many complex issues, the bulletin concludes, "There are no simple rules for specifying when to use a blue or green backing. Each situation must be analyzed to see whether a blue or a green backing will yield better results."
Among the simplest of techniques for achieving a bluescreen element is that of deploying a fabric backing of the appropriate chrominance and luminance and staging the scene before it. This, indeed, has been one of the principal methods employed for several years. When it is possible to isolate the lighting of the backing from the lighting of the foreground scene, it is possible to achieve excellent results. The author's developments of fluorescent light sources specific to the task (as cited in the ASC Manual, p. 435) and those of others in the field have greatly improved the results obtained by this approach. However, it is increasingly desirable to be able to place the foreground action directly in, or on, the backing. In this situation the same light will, of necessity, light both the backing and the foreground action. As the discussion on page 436 of the ASC Manual illustrates, the existing techniques employing fabrics of the prior art are far from effective. Painted backings and floors have yielded better results, as these have been possible to endow with enhanced properties versus fabric. The greatest success in this approach has been the employment of fluorescent pigments incorporated in both opaque and transparent paints, (some aspects of this discussion are described in U.S. Pat. No. 4,417,791), as these permit greater chromatic actinic stimulus for photographic film than do conventional pigments.
However, painted backings suffer the disadvantage of the relatively high cost of providing an appropriate substrate, the very high cost of the pigments required and the labor to apply them, as well as the inordinate time required for the whole operation. To obtain the efficacy of high quality painted backings with the simplicity, speed and economy of fabric backings requires the development of a new type of dyed fabric backing.